This invention relates to syringes, and more particularly, to a plunger having an improved sealing ability for use with a syringe.
Syringes are devices routinely employed for providing a quantity of a medical liquid to a patient, typically into the patient""s arteriovenous system, and for dispensing liquids in other non-medical applications. Syringes are traditionally configured with a tubular barrel for holding the liquid, a plunger within the barrel, an access opening for the plunger at a proximal end of the barrel, and a smaller discharge outlet at a distal end of the barrel. The plunger has a proximal portion adapted to receive a dispensing force, a central section with sealing flanges, and a distal head that contacts the liquid within the barrel. During a dispensing procedure, a dispensing force moves the plunger in a distal direction along the longitudinal length of the barrel, if the magnitude of the dispensing force is sufficient, to urge the liquid contained in the barrel through the discharge outlet. The sufficiency of the dispensing force depends, among other factors, upon the viscosity of the liquid held by the barrel.
Plungers usually carry two annular sealing flanges of substantially identical dimension and in a spaced relationship that extend about the circumference of an outer surface of the plunger. Each flange has a barrel-contacting sealing surface configured to compressively engage the interior of the barrel. An annular chamber is defined in the space bounded by the adjacent sealing flanges, the cylindrical side wall of the plunger and the interior cylindrical surface of the barrel. During a dispensing procedure, the distal sealing flange exerts a contact pressure against the interior wall of the barrel sufficient to prevent unwanted leakage of the liquid past the plunger in a proximal direction. The proximal sealing flange is present to ensure proper alignment of the plunger during a dispensing procedure and to prevent the passage of air during when the syringe is aspirated to, for example, fill the interior of the barrel forward of the plunger with liquid.
As the dispensing force advances the plunger, the liquid in the barrel exerts a significant hydrostatic pressure against a distal surface of the plunger. The plunger is formed of a resilient material that responds to the hydrostatic pressure by compressing along its longitudinal axis. The axial compression is accommodated by an outward distention of the plunger body. The sealing flanges likewise move radially outward so that the contact pressure is enhanced between the barrel-contacting sealing surface of the distal sealing flange and the interior of the barrel. The enhanced contact pressure offsets the increased hydrostatic pressure exerted by the liquid against the distal sealing flange and preserves the integrity of the fluid-tight barrel-contacting engagement. However, the outward distension of the side walls also reduces the volume of the annular chamber.
The annular chamber is normally occupied by a compressible fluid, such as air, that can accommodate a change in volume without supplying a significant opposing force against the contact pressure. In certain situations, however, a quantity of liquid may pass the distal sealing flange and become trapped in the annular chamber nearest to the distal sealing flange prior to either initiating or completing the dispensing procedure. For example, liquid may become trapped in the annular chamber when the syringe is filled or if the plunger cants during the dispensing procedure. Liquids are relatively incompressible fluids that occupy roughly a constant volume independent of an applied external pressure.
If a significant quantity of liquid is trapped in the annular chamber, the contact pressure exerted against the interior of the barrel by the sealing surfaces of the sealing flanges will be reduced. In particular, the distal sealing flange may actually exert a lesser contact pressure against the interior of the barrel than the proximal sealing flange. During the dispensing operation, the contact pressure between the distal sealing flange and the interior of the barrel cannot increase commensurate with the axial compression due to the presence of the incompressible trapped liquid. As a result, the sealing ability of the distal sealing flange may be compromised.
If the distal sealing flange loses contact with the interior of barrel, the highly pressurized liquid within the barrel will rush through the newly-created gap. The proximal sealing flange is not configured to resist the flow of highly pressurized liquid and readily yields to the leaking liquid, which exits past the plunger in a proximal direction toward the access opening. When such xe2x80x9cblow-byxe2x80x9d events occur, significant quantities of liquid can stream past the plunger. Among the undesirable side effects of blow-by, the patient will receive less liquid per unit distance of plunger movement than the prescribed dosage. If the syringe is recycled to perform multiple dispensing operations, the likelihood of a blow-by event increases with each subsequent dispensing operation.
Liquid may be dispensed in an precise fashion over a lengthy duration with the assistance of a power injector. Power injectors utilize a motor-driven plunger drive adapted to engage and continuously advance the plunger for incrementally dispensing the contents of the syringe over an extended time according to predetermined injection parameters such as flow rate, volume, duration, and time. Power injectors are commonly used for carrying out extended infusions of a liquid, such as an imaging contrast agent, into the vascular system of a patient, because of the greater reliability and consistency in infusion rates and dosage when compared to a manual injector. The occurrence of blow-by in a syringe being actuated by a power injector is more deleterious that blow-by associated with a manual injection because the injection pressure can approach 1200 pounds per square inch (psi). However, blow-by has been noted for injection pressures as low as 50 psi. In addition to the uncertainties in patient dosage discussed above, the power injector itself can be damaged by the introduction of liquid into the mechanism or by the need to overdrive the motor to compensate for a reduced delivery rate. Further, the injector and the portion of the procedure room near the injector may be soiled by the escaped liquid. Still further, the sterility of the equipment may be compromised.
Thus, an improved plunger is needed for a syringe having a configuration of sealing flanges that will prevent blow-by of the liquid held within the syringe barrel during a dispensing procedure and that can do so without significantly increasing the sliding frictional force between the sealing flanges and the interior of the barrel.
The present invention addresses these and other problems by defining a plunger with dual sealing flanges for use with a syringe, where the plunger is configured to prevent a blow-by event. Further, the present invention provides a plunger configured with a pressure relief mechanism that preferentially ejects any liquid trapped between the sealing flanges of the plunger so as to prevent blow-by. The present invention provides sealing flanges with optimized dimensions or modified structure that eliminates blow-by without significantly altering the frictional force between the sealing flanges and the interior of the syringe barrel. Further, the present invention provides a plunger that is not sensitive to trapped liquid so that the more complex factors that contribute to the trapping of the liquid do not have to be addressed and solved.
According to the present invention, one embodiment of the plunger has a piston with a chamber susceptive of trapping a portion of the liquid when the plunger is in motion. When the plunger is moved by a dispensing force in a proximal-to-distal direction to dispense liquid from the syringe, the piston is configured to maintain a fluid-tight contact pressure with the interior surface of the syringe barrel and to preferentially exhaust any trapped liquid in a proximal direction.
In another embodiment, the piston has first and second spaced circumferential sealing flanges that project radially outwardly from a cylindrical outer surface. The second or proximal sealing flange is spaced from the first sealing flange to define a chamber, which may have an annular configuration. The first or distal sealing flange is substantially circumferentially continuous about the cylindrical outer surface of the plunger and has a barrel-contacting sealing surface that exerts a first contact pressure on the interior surface of the syringe barrel sufficient to normally maintain a fluid-tight seal therewith. The second sealing flange likewise has a barrel-contacting sealing surface configured to exhaust liquid trapped in the chamber in a proximal direction. The piston has a head, that is usually conical and integral with the cylindrical outer surface, that contacts the liquid held by the barrel and, according to one embodiment, that tapers to an included angle of about 120xc2x0.
According to one embodiment of the present invention, the second sealing flange may be substantially circumferentially continuous and exert a second contact pressure on the inside surface of the barrel that is lesser than the first contact pressure exerted by the first sealing flange. As a result, any liquid trapped in the chamber preferentially overcomes the second contact pressure and is exhausted in a proximal direction without disturbing the fluid-tight seal provided by the first sealing flange. The differential in contact pressure may be established by mismatching the radial distance of the sealing flanges, measured relative to the longitudinal axis of the barrel, so that the first sealing flange has a larger radial distance than the second sealing flange. Increasing the radial distance of the first sealing flange by 10 mils has been found to optimize the pressure relief without significantly altering the sliding frictional forces between the sealing flanges and the interior of the barrel.
In another embodiment, the second sealing flange is a circumferential sealing lip that projects in a radially outward and proximal direction from the cylindrical outer surface of the plunger. The sealing lip has a barrel-contacting sealing surface that contacts the inside flange of the syringe barrel. The sealing surface exerts a lesser contact pressure with the interior of the barrel than the first sealing flange and, as a result, the sealing lip can deflect proximally to exhaust any pressurized fluid trapped in the chamber between the sealing flanges without compromising the sealing ability of the first sealing flange.
In another embodiment, the plunger may comprise an outer sheath that surrounds the exterior of an inner member. The sheath includes first and second spaced circumferential sealing flanges that project radially outwardly from an exterior surface of a cylindrical sidewall for compressively engaging the interior of the syringe barrel.
In other embodiments, the contact pressure exerted by the second sealing flange may be reduced by removing a portion of the exterior of the piston or by removing a portion of the interior of the sheath that, in each case, is radially inwardly of the second sealing flange. The reduction in the contact pressure exerted by the second sealing flange against the interior of the barrel relative to the contact pressure of the first sealing flange provides a path of lesser resistance for pressurized trapped fluid to exhaust in a proximal direction.
The present invention shall become more apparent from the accompanying drawings and description thereof.